1. Field of the Invention
This invention relates generally to acoustic surface wave devices and more particularly to techniques for synthesizing desired device frequency characteristics. The desired frequency characteristics are provided in the present invention by appropriate phase-encoding of the transducer elements as contrasted with the amplitude-encoding of the transducer elements of the prior art.
2. Description of the Prior Art
In acoustic surface wave devices, varying electric signals, applied to properly-positioned transducer electrodes deposited on a piezoelectric substrate, generate acoustic waves which propagate along the substrate surface. Correspondingly, the passage of acoustic waves on the surface of a piezoelectric substrate through a region in which properly-positioned transducer electrodes have been deposited, will generate an electric signal at the output of the transducer. These phenomena can be used to provide a group of electronic devices having predetermined transfer-function characteristics.
To provide specified frequency characteristics in an acoustic surface wave device, the desired characteristics, which are typically specified in the frequency domain, are transformed into a time domain representation. The time domain representation is chosen because of the correspondence to the physical situation in which an acoustic wave propagates with a velocity characteristic of the device substrate and interacts with the electro-acoustic elements of a transducer in an acoustic surface wave device. The propagation of the acoustic surface wave in the device corresponds to propagation along the time axis in the time domain representation.
The characteristics of the transducer elements, with which the propagating acoustic surface wave interacts, are chosen to produce an electrical analog of the function displayed in the time domain representation. In particular, one set of transducer elements, assigned an arbitrary polarity, is associated with the local time domain maxima, while a second set of transducer elements, arbitrarily assigned the opposite polarity, is associated with the local time domain minima. The magnitude of local maxima and minima of the time domain representation is reproduced in the acoustic surface wave device by appropriate amplitude of overlap between adjacent transducer elements of the two transducer elements sets, resulting in an amplitude-encoding of the transducer elements.
In reproducing the characteristics specified in the time domain representation, the interactive region of the acoustic surface wave transducer (i.e., the region of element overlap) typically increases from zero, reaches a maximum and decreases to zero. Thus a transducer, when used to generate acoustic surface waves, provides a relatively diffuse wave pattern. The generation of acoustic surface waves is therefore generally restricted in the prior art to a series of uniformly overlapping transducer elements to provide a relatively narrow region of propagation of the surface waves about a center frequency and is not suitable for imposition of specified characteristics. Such characteristics can typically be imposed only on the output transducer of an acoustic surface wave device.
It is therefore an object of the present invention to provide an improved acoustic surface wave device.
It is another object of the present invention to provide a method of modulating an acoustic surface wave device.
It is further object of the present invention to provide a method of achieving specific frequency characteristics for a device transfer function.
It is a still further object of the present invention to provide a method of phase-encoding of transducer elements in an acoustic surface wave device.
It is yet another object of the present invention in which desired frequency characteristics can be specified for both an input and an output transducer of the acoustic surface wave device.
It is still another object of the present invention to provide a method of generating acoustic surface waves having specified frequency characteristics in which the direction of propagation of the surface waves is approximately perpendicular to the transducer elements.
It is a more particular object of the present invention to provide acoustic surface wave transducers which produce signal components, generated by an associated transducer element group, having specified characteristics.
It is yet another particular object of the present invention to provide an acoustic surface wave device in which a phase-encoded electro-acoustic transducer is functionally equivalent to an amplitude-encoded electro-acoustic transducer.
It is still another object of the present invention to control the phase of a signal associated with an element of an electro-acoustic transducer by altering the element position.
It is yet a further object of the present invention to alter the phases of signals associated with elements of an electro-acoustic transducer, wherein a combination of the phase-altered signals produces a signal of preselected phase and amplitude.